Implement getgroupmembership(3). This is similar to getgrouplist(3), except
that the "int maxgroups" and "int *ngroups" parameters are separated into
two separate parameters which makes it possible to call multiple nsswitch
back-ends and have the results correctly merged.
getgrouplist(3) is now implemented using getgroupmembership(3).
Proposed on tech-userlevel on December 1, 2004.